The "black pad" phenomenon, which refers to the blackening of electroless-plated nickel-phosphorus [Ni(P)] films during the immersion Au process, is reproduced using pure chemicals and its fundamental mechanisms are investigated. In the present analysis, under bump metallurgy (UBM) materials have profound effects on the black pad susceptibility, and the presence of abnormally large nodules (ALNs) is essential to the black pad occurrence. The Ni(P) films over Cu, Ag, and Au substrates all exhibit ALNs and are susceptible to black pads, while those over Ni and Co substrates do not have ALNs and therefore are not susceptible to black pad. In the former cases, submicron scale nodular variations of the surface curvature lead to variations in the P concentration in the Ni(P) films, which induces sufficiently large potential differences to drive galvanic corrosion when exposed to the electrolyte, which is a gold cyanide solution in this study. The UBM effect is ascribed to differences in the Ni(P) film growth mode, where the transition from a layer-by-layer growth mode to an island growth mode is easier over Cu, Ag, and Au UBMs.